Known imaging techniques with tremendous importance in medical diagnostics are positron emission tomography (PET), computed tomography (CT), magnetic resonance imaging (MRI), single photon computed tomography (SPECT) and ultrasound (US). Although today's imaging technologies are well developed they rely mostly on non-specific, macroscopic, physical, physiological, or metabolic changes that differentiate pathological from normal tissue.
Targeting molecular imaging (MI) has the potential to reach a new dimension in medical diagnostics. The term “targeting” is related to the selective and highly specific binding of a natural or synthetic ligand (binder) to a molecule of interest (molecular target) in vitro or in vivo.
MI is a rapidly emerging biomedical research discipline that may be defined as the visual representation, characterization and quantification of biological processes at the cellular and sub-cellular levels within intact living organisms. It is a novel multidisciplinary field, in which the images produced reflect cellular and molecular pathways and in vivo mechanism of disease present within the context of physiologically authentic environments rather than identify molecular events responsible for disease.
Several different contrast-enhancing agents are known today and their unspecific or non-targeting forms are already in clinical routine. Some examples listed below are reported in literature.
For example, Gd-complexes could be used as contrast agents for MRI according to “Contrast Agents I” by W. Krause (Springer Verlag 2002, page one and following pages). Furthermore, superparamagnetic particles are another example of contrast-enhancing units, which could also be used as contrast agents for MRI (Textbook of Contrast Media, Superparamagnetic Oxides, Dawson, Cosgrove and Grainger Isis Medical Media Ltd, 1999, page 373 and following pages). As described in Contrast Agent II by W. Krause (Springer Verlag 2002, page 73 and following pages), gas-filled microbubbles could be used in a similar way as contrast agents for ultrasound. Moreover “Contrast Agents II” by W. Krause (Springer Verlag, 2002, page 151 and following pages) reports the use of iodinated liposomes or fatty acids as contrast agents for X-Ray imaging.
Contrast-enhancing agents that can be used in functional imaging are mainly developed for PET and SPECT.
The application of radiolabelled bioactive peptides for diagnostic imaging is gaining importance in nuclear medicine. Biologically active molecules which selectively interact with specific cell types are useful for the delivery of radioactivity to target tissues. For example, radiolabelled peptides have significant potential for the delivery of radionuclides to tumours, infarcts, and infected tissues for diagnostic imaging and radiotherapy.
DOTA (1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10tetraazacyclododecane) and its derivatives constitute an important class of chelators for biomedical applications as they accommodate very stably a variety of di- and trivalent metal ions. An emerging area is the use of chelator conjugated bioactive peptides for labeling with radiometals in different fields of diagnostic and therapeutic nuclear oncology.
There have been several reports in recent years on targeted radiotherapy with radiolabeled somatostatin analogs.
US2007/0025910A1 discloses radiolabled somatostatin analogs primarily based on the ligand DOTA-TOC. The radionucleotide can be (64)Copper and the somatostatin analog may be octreotide, lanreotide, depreotide, vapreotide or derivatives thereof. The compounds of US2007/0025910A1 are useful in radionucleotide therapy of tumours.
US2007/0025910A1 does not disclose (64)Cu-DOTA-TATE. DOTA-TATE and DOTA-TOC differ clearly in affinity for the 5 known somatostatin receptors (SST1-SST2). Accordingly, the DOTA-TATE has a 10-fold higher affinity for the SST2 receptor, the receptor expressed to the highest degree on neuroendocrine tumors. Also the relative affinity for the other receptor subtypes are different. Furthermore, since 177Lu-DOTATATE is used for radionuclide therapy, only 64Cu-DOTATATE and not 64Cu-DOTATOC can be used to predict effect of such treatment by a prior PET scan.
There exists a need for further peptide-based compounds having utility for diagnostic imaging techniques, such as PET.